Method of dispensing fluid onto a wafer

ABSTRACT

A method for dispensing fluids on a semiconductor wafer wherein a wafer is mounted on a rotatable chunk, a fluid to be dispensed is introduced into a well through an inlet located adjacent the bottom of the well, rotating the chunk and moving a soft inpact dispensing nozzle, that draws bubble-free fluid from the bottom of the well, over the wafer, and dispensing the fluid at a low pressure and a short distance to the wafer surface.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a coating apparatus for applying aliquid to a semiconductor wafer, and more particularly to a spin coatingapparatus for applying a thin uniform thickness coating of a photoresistor developer to a semiconductor wafer.

2. Description of the Related Art

As the degree of miniaturization of semiconductor devices increases, thecontrol of the thicknesses of photoresists, developer, etc. becomesincreasing critical. Also the prevention of the formation of bubbles iscritical. It is conventional to apply coatings on wafers by spraying theliquid resist or developer over a wafer mounted on a spindle as it isrotated. A typical prior art apparatus is illustrated in FIG. 1. A wafer10 is shown mounted on a rotation spindle 12 as developer solution 14 orother liquid is dropped or sprayed on the wafer from nozzle 16. Line 18supplies the liquid to the nozzle 16. The temperature of the liquid canbe controlled by a suitable heat exchange apparatus, not shown, whichmaintains the liquid at a uniform desirable temperature to achieve adesired viscosity.

Current design in semiconductor manufacture normally has a nozzlelocated at an extreme height, which results in a relatively high impactof the liquid onto the product wafer. This results in poor criticaldimensions of the coating geometries across the wafer. Also bubbleformation, due to the alkaline nature of the developer liquid, is verydominant, and can also cause an impact on the developer uniformityperformance. Even if the nozzle is simply moved near to the surface ofthe substrate or wafer, the problems are not solved due to the highpressure of impact of the solution onto the wafer. The pressure of thedevelopment fluid onto the wafer can be as high as 6 psi., because ofthe needed pressure to dispense the fluid. FIG. 4 shows the effect ofthe prior art process at curve 80 in the plot of Critical Dimension (CD)versus wafer radius after the development process. It shows the greaterdevelopment of the photoresist at the center of the wafer than at thesides of the wafer. This is also better understood with reference toFIG. 5 wherein the result of this over development at the center, usinga typical 1.0 micrometer feature size results in photoresist mask of thedesired 1.0 micrometer lines 84 at the edges of the wafer 10 and theundesired (too narrow) 0.8 micrometer lines at the center of the wafer.It is seen that curve 82, which is the result of the present newapparatus and method as described below would produce uniform maskdimensions across the wafer.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an improved spincoating apparatus for use in the semiconductor fabrication industry thatwill lay down a coating layer that is more uniform in thickness andsubstantially free from bubbles.

Another object of the invention is to provide a spin coating apparatusthat provides a soft impact with low bubble density of liquid coatingmaterial on a wafer surface.

In accordance with the afore-mentioned objectives, there is provided aspin coating apparatus for applying a liquid material to a semiconductorwafer, or the like, that has a spin head for supporting a wafer on thetop surface, a motor to rotate the spin head, and a nozzle located overthe spin head for dispensing liquid on the wafer mounted on the spinhead. A liquid well is provided having a bottom outlet opening, a bottominlet opening, a top vent opening, and a heat exchange jacket on atleast the well side walls. A shut-off valve is located between thenozzle and well, with the inlet of the valve communicating with thebottom outlet of the well, and the inlet of the valve communicating withthe nozzle. A liquid supply source provides liquid to the well throughthe inlet opening of the well. A multi-stage vent and purge system isprovided to vent and selectively introduce either gas or liquid cleaningmedium into the well through the top vent opening. The distance betweenthe nozzle and the spin head is controlled with a nozzle support means.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a typical spin-coating apparatus that iswell known in the Prior Art.

FIG. 2 is a schematic view of the essential elements of the coatingapparatus of the invention.

FIG. 3 is a front view of a preferred embodiment of the coatingapparatus of the invention which shows other essential elementsassociated in the combination.

FIG. 4 is a graph showing the results of the Prior Art and the presentnovel method and apparatus after the developing process in terms ofCritical Dimension (CD) versus wafer radius.

FIG. 5 is a schematic cross-sectional representation of the result ofthe Prior Art development of the photoresist mask and etching processusing the Prior Art apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Various embodiments of the coating apparatus in accordance with thepresent invention will be described with reference to the accompanyingdrawings.

Referring now to FIG. 2, there is depicted a first preferred embodimentof the invention. The coating apparatus has a conventional spin head 20mounted for rotation about a vertical axis by spindle 22. Asemiconductor wafer 24 is temporarily mounted on spin head 20, as iswell known. A suitable motor, not shown, is provided to rotate thespindle at a suitable speed, normally in the range of 5 to 5000revolutions per minute. Above the spin head 20, is provided a nozzle 26for dispensing liquid, such a photoresist solution, developer solution,or the like, onto the wafer 24 as it is rotated.

The nozzle 26 tip is tapered to provide a laminar flow of the developerfluid. The preferred location of the nozzle is about 5 to 10 millimetersoff-center as show in FIG. 3. FIG. 2 shows a centrally located to wafernozzle.

A liquid well 28 is provided above the nozzle. The well 28 has a bottomoutlet opening 30, a bottom inlet opening 32 to introduce liquids intothe well, a top vent opening 34, and a heat exchange jacket 36 tomaintain the liquid in the well at a suitable temperature. Thetemperature of the liquid will depend on the type of solution and theparticular application. In general the temperature of a photoresistsolution should be in the range of 20° to 24° C. depending on the waferFAB cleanroom temperature. The temperature of a developer solutionshould be in the range of 22° to 23° C. The control of the developerfluid is very essential to the development process of photoresist. Anintermediate developer wall 38 within well 28 is provided to facilitatea controlled temperature. Cooled and temperature controlled water ispumped through the wafer jacket between well 28 and well 38 to sustain acontrolled temperature for the developer solution in the well 28. Sincethe developer performance decreases with increase in temperature as iswell known. A thermocouple junction (not shown) is positioned at thislocation to monitor the temperature and to allow the temperature to bemaintained at the desired number. Both wells can be formed of aluminumto enhance the heat transfer.

Mounted between nozzle 26 and well 28, is a conventional Teflon shut-offvalve 40 in communication with the outlet opening 30 of well 28, and theinlet of nozzle 26. Valve 40 is activated by pneumatic signal generatedfrom a 3-2 way solenoid valve which in turn is triggered from anelectrical signal generated from an output port of some CPU or logiccircuit.

A liquid supply is connected to pipe 42 to introduce liquid into thebottom of well 28, through inlet opening 32. Preferably the liquid isintroduced into the well at a pressure in the range of 6 to 11 psi.Valve 44 is provided to control the flow of liquid. This a criticalfeature of the invention. The well 28 avoids the direct dispense ofdeveloper solution, which could be in the range of 6 to 11 psi pressurehead onto the wafer. This indirect dispense allows the dispense ofdeveloper solution at a controlled 1 to 3 psi. onto the wafer. This is asoft impact dispense which gives the excellent Critical Dimension (CD)curve 82 shown in FIG. 4.

The developer solution supply is connected to pipe 42 to introducesolution into the bottom of the well 28, through inlet opening 32. Thissolution pressure is usually at a range of 6 to 11 psi. depending uponthe pump used to transfer the solution. Valve 44 is provided to controlthe flow of the liquid.

As more clearly shown in FIG. 3, nozzle 26, shut-off valve 40, and well28 are preferably mounted as a unit on support 46. This unit is movedvertically into and out of position over wafer 24, by a suitablecylinder 48 or the like. Arrow 50 indicates the vertical movementprovided by cylinder 48.

As shown in FIG. 2, a multi-stage vent and purge system is provided forventing and selectively introducing a gas or liquid cleaning medium intowell 28 through vent opening 34. A master shut-off valve 52 in line 54controls the entire vent and purge operation. A atmosphere valve 56 inline 58 and metering valve 60 permits venting of the well 28.

The operation of the system is as follows. When the dispense of thedeveloper onto the wafer is not as yet needed, the valve 44 will open toallow the relatively high pressured developer (6 to 11 psi) into thewell 28. The valve 40 will be closed while valves 52, 56 an 62 will beopened. Metering valves 60 and 66 allows the control of the rate ofventing of the well 28 depending on the requirements of the process.Such requirements are, for example the cycle of development processwhich if it is a short process--then a high venting is needed to allowthe well to fill up faster to cater to the short cycle of each wafer asdeveloped. The duration of valve being open and allowing the developersolution to enter the well 28 can be controlled by a timer or anothersensor (not shown) sensing the presence of the solution in line 54. Whenthe well 28 is full, overflowing solution will travel in line 54. Sensorlocated therein could then trigger shut off valve 44 and valve 52.

In like manner, gas valve 62 in line 64, and metering valve 66 permitsventing of well 28 of non-atmosphere gas, such as nitrogen, helium, etc.

Purge valve 68 in line 70, and check valve 72 permits introducingliquids such as de-ionization (DI) water, or inert gases into well 28,in combination with 3 way valve 74. The purge line 70 will be used afterthe well 28 has been emptied or almost total dispense of the developersolution onto the semiconductor wafer has occurred. There is residualdeveloper solution left in the well 28. During rinsing cycle, valves 74,68 will be activated to allow deionized (DI) water to flush out theremaining solution in the well 28. Nitrogen (N₂) purge can also beintroduced after DI water purge to dry the nozzle 26 to preventdripping.

The method of operation, which avoids the direct dispensing of developersolution at 6 to 11 psi. pressure head onto the wafer, uses techniqueswhich allows a controlled dispensing pressure of between about 1 to 3psi. and is as follows. The intermediate developer solution well 28 isfilled by introducing the fluid into the bottom of the well so as toreduce the possible formation of bubbles at the normal supply pressureof 6 to 11 psi. The filling of the well is done when the nozzle 26 is inits uppermost position as indicated by arrow 50 in FIG. 3 and ascontrolled by means 48. The developer solution or fluid supply isconnected to pipe 42 to introduce solution into the bottom of well 28through inlet opening 32. Valve 40 will be closed while valves 52, 56and 62 will be open. Metering valves 60, 66 allows the control of therate of venting of well 28 depending on the requirements of, for examplethe cycle of developing process which if it is a short cycle process,then a high venting is needed to allow the well 28 to fill up faster tocater to the short cycle of each wafer being developed.

The duration of opening of valve 44 for filling of the well 28 can becontrolled by timer or other sensor (not shown) sensing the presence ofsolution in line 54. When well 28 is full, overflowed solution willtravel in line 54. The sensor located therein will then trigger the shutoff of valve 44 and valve 52. With the well 28 full, it is preferred toallow the apparatus to sit as is for as long as possible to allow anybubbles in the well fluid to rise to the top.

A new wafer 24 is now moved onto the wafer rotating chuck 20 and underthe well 28 and nozzle 26 while they are at their uppermost position.When the developer solution is called to be dispensed onto the wafer 24,the whole well 28 and nozzle 26 apparatus is moved down to just abovethe surface of the wafer 24 as seen in FIGS. 2 and 3. The criticalheight of the tip of the nozzle 26 above the wafer 24 is between about 5to 15 millimeters. The nozzle is preferably off-center as seen in FIG. 3by between about 5 to 10 millimeters.

As can now be seen best with reference to FIG. 2, the release of thedeveloper solution or fluid onto the wafer 24 is done by opening valves40 and 52 to line 54. Metering valve 60 can be preset to control therate of venting. With this condition, the developer solution in well 28will flow gently onto the wafer 24 at a controlled pressure of betweenabout 1 to 3 psi. under this controlled venting. A close loop control isanother possibility for enhanced performance. The features of thetapered well 28 and tapered nozzle 26 add to the gentle draining of thefluid by laminar flow through the valve 40. At the end of the dispensingstep, the valve 40 is closed. The well 28 and nozzle 26 apparatus israised to its uppermost position.

The photoresist on the wafer is developed while it is still on the waferchuck 20 without spinning. This takes about 40 to 60 seconds. At the endof the timed process, the remaining developer fluid on the wafer is spunoff of the wafer by spinning the chuck. The wafer is rinsed with DIwater spray from DI water spray means 86. During this rinsing step, thevalves 40 and 52 will open again along with 3-2 way solenoid valve 74will trigger to allow the DI water to purge through lines 70 and 54 intowell 28 and valve 40 hence cleaning the path. This prevents a long termclogging. If necessary, nitrogen can then be introduced through the samelines to dry these lines. The wafer 24 is now moved off the chuck 20 forfurther processing. The process now will repeat itself for a new wafer.

The advantages of the new apparatus and method for dispensing fluid upona wafer include the provision of a soft impact fluid dispension whichenhances the control of the resulting critical dimension across thewafer, the provision of a low bubble resist developer on the wafer and acleaned in process nozzle.

While the invention has been particularly shown and described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art, that various changes in form and details maybe made without departing from the spirit and scope of the invention.

What is claimed is:
 1. A method for dispensing a developer fluid onto awafer having a resist coating to be developed supported on rotatingchuck in a soft impact manner comprising:filling a developer well whichis associated with a soft impact tapered nozzle by introducing thedeveloper fluid into said well through an inlet located adjacent thebottom of said well so as to reduce the possible formation of bubbles ata normal pressure of between about 6 and 11 psi.; moving said wafer ontosaid chuck and under said soft impact nozzle; rotating said chuck;dispensing said developer fluid, while said chuck is rotating from saidwell through said nozzle onto the central area of said wafer undercontrolled pressure of less than 3 psi and in said soft impact manner;stopping the flow of said developer fluid; developing said resist onsaid wafer without rotating said chuck, while said wafer is on saidchuck; removing unreacted developer fluid by rotating said chuck and byrinsing and drying said resist; and removing said wafer from said chuck.2. The method of claim 1 wherein said well and said nozzle are cleanedand rinsed of developer fluid with the liquid which does the cleaningand rinsing at the same time as said rinsing and drying with said liquidflowing onto said wafer.
 3. The method of claim 1 wherein a shut-offvalve for dispensing said developer fluid and said well are mounteddirectly above said nozzle.
 4. The method of claim 1 wherein an inletline is provided to said well that enters the well through said inletlocated on the bottom of said well so as to reduce bubble formation insaid well.
 5. The method of claim 1 wherein the distance between saidnozzle and said wafer is between about 5 to 15 millimeters for applyingsaid developer to said wafer at soft impact.
 6. The method of claim 1wherein said developer fluid is dispensed from the nozzle spacedoff-center from the center of said wafer, a distance in the range of 5to 10 millimeters.
 7. A method for developing a resist on a wafersurface in a controlled and uniform manner across the wafercomprising:filling an intermediate developer well which is associatedwith and directly above a soft impact tapered nozzle by introducing thedeveloper fluid into the bottom of said well so as to reduce thepossible formation of bubbles at a normal pressure of between about 6and 11 psi.; moving said wafer having a resist coating thereon onto arotatable chuck and directly under said soft tapered impact nozzle;rotating said chuck; dispensing said developer fluid, while said chuckis rotating, from said intermediate well through said nozzle locatedbeneath said well onto the central area of said wafer under controlledand said soft impact manner at a controlled pressure of less than about3 psi.; stopping the flow of said developer fluid; developing saidresist on said wafer without rotating said chuck, while said wafer is onsaid chuck; removing unreacted developer fluid by rotating said chuckand by spraying a cleaning liquid at said wafer for rinsing saiddeveloper from said wafer and simultaneously rinsing said developer fromsaid well and nozzle onto said wafer; and removing said wafer from saidchuck.
 8. The method of claim 7 wherein said distance between said waferand nozzle is between about 5 to 15 millimeters for applying saiddeveloper to said wafer at soft impact.